
# 🍔 Domain Driven Design

For many architects, the process of data modeling is driven by intuition. However, there are well-formulated methodologies
for approaching it more formally. I recommend the [Domain-Driven Design](https://en.wikipedia.org/wiki/Domain-driven_design)
and choose it as AppFlowy architecture.

## 💥 Layered architecture
The most common architecture pattern is the layered architecture pattern, known as the n-tier architecture pattern.
Partition the software into `layers` to reduce the complexity. Each layer of the layered architecture pattern has a
specific role and responsibility.`DDD` consists of four layers.

```
    ┌──────────────────────────────────────────────────┐         ─────────▶
    │                Presentation Layer                │──┐      Dependency
    └──────────────────────────────────────────────────┘  │
                              │                           │
                              ▼                           │
    ┌──────────────────────────────────────────────────┐  │
    │                Application Layer                 │  │
    └──────────────────────────────────────────────────┘  │
                              │                           │
                              ▼                           │
    ┌──────────────────────────────────────────────────┐  │
    │                   Domain Layer                   │◀─┘
    └──────────────────────────────────────────────────┘
                              ▲
                              │
    ┌──────────────────────────────────────────────────┐
    │               Infrastructure Layer               │
    └──────────────────────────────────────────────────┘
```

**Presentation Layer**:
* Responsible for presenting information to the user and interpreting user commands.
* Consists of Widgets and also the state of the Widgets.

**Application Layer**:
* Defines the jobs the software is supposed to do. (Shouldn't find any UI code or network code)
* Coordinates the application activity and delegates work to the next layer down.
* It doesn't contain any complex business logic but the basic validation on the user input before
  passing to the other layer.

**Domain Layer**:
* Responsible for representing concepts of the business.
* Manages the business state or delegated to the infrastructure layer.
* Self contained and it doesn't depend on any other layers. Domain should be well isolated from the
  other layers.

**Infrastructure Layer**:

* Provides generic technical capabilities that support the higher layers. It deals with APIs, persistence and network, etc.
* Implements the repository interface and hides the complexity of the Domain layer.

As you see, the `Complexity` and `Abstraction` of these layers are depicted in the following diagram. Software system are composed in layers,
where higher layers use the facilities provided by lower layers. Each layer provides a different abstraction from the layer above
and below it. As developers, we should pull the complexity downwards. Simple interfaces and powerful implementations (Think about the
[open](https://man7.org/linux/man-pages/man2/open.2.html) function). Another way of expressing this idea is that it is more important
for a module to have a simple interface than a simple implementation.


```
                 ▲
                 │
    Level of     ├───────────────────┐
    Abstraction  │ Presentation      │
                 ├───────────────────┴───────┐
                 │ Application               │
                 ├───────────────────────────┴─────────┐
                 │ Domain                              │
                 ├─────────────────────────────────────┴────────┐
                 │ Infrastructure                               │
                 └──────────────────────────────────────────────┴─────▶
                                                           Complexity
```


### Data Model
DDD classifies data as referenceable objects, or entities, and non-referenceable objects, or value objects. Let's introduce some DDD terminology.

**Entity**

`Entities` are plain objects that carry an identity which allows us to reference them. e.g. user, order, book, etc.
You use `entities` to express your business model and encapsulate them into Factory that provides a simple API to create Entities.

**Value Object**

`Value Object` can't be referenced. They can be only included into entities and serve as attributes. Value objects could be simple and treat as immutable. e.g. email, phone number, name, etc.

**Aggregate**

`Entities` and `Value objects` can be grouped into aggregates. Aggregates can simplify the model by accessing the entire aggregate.
For instance, Table has lots of rows. Each row using the table_id to reference to the
table. TableAggregate includes two entities: Table and the Row.

```
     TableAggregate
    ┌────────────────────────────────────────────────────────────────┐
    │                                                                │
    │  ┌────────────────────┐         ┌─────────────────────────┐    │
    │  │struct Table {      │         │struct Row {             │    │
    │  │    id: String,     │         │    table_id: String,    │    │
    │  │    desc: String,   │◀▶───────│}                        │    │
    │  │}                   │         │                         │    │
    │  └────────────────────┘         └─────────────────────────┘    │
    │                                                                │
    └────────────────────────────────────────────────────────────────┘
```

**Service**

When a significant process of transformation in the domain is not a natural responsibility of an `Entity` or `Value object`, add an operation to the model as standalone interface declared as a Service. For instance: The `Value object`  EmailAddress uses the function `validateEmailAddress` to verify if the email address is valid or not. `Service` exists in Application, Domain and Infrastructure.

```
class EmailAddress  {
  final Either<Failure<String>, String> value;

  factory EmailAddress(String? input) {
    return EmailAddress._(
      validateEmailAddress(input),
    );
  }

  const EmailAddress._(this.value);
}


Either<Failure<String>, String> validateEmailAddress(String? input) {
  ...
}
```

**Repository**

Repositories offer an interface to retrieve and persist aggregates and entities. They hide the database or network details from the domain.

Repository interfaces are declared in the Domain Layer, but the repositories themselves are implemented in the Infrastructure Layer.
You can replace the interface implementation without impacting the domain layer.

For instance:

```
// Interface:
abstract class AuthInterface {
    ...
}

// Implementation
class AuthRepository implements AuthInterface {
    ...
}
```
> More often than not, the repository interface can be divided into sub-repository in order to reduce the complexity.

### Relation
The diagram below is a navigational map. It shows the patterns that form the building blocks of Domain Driven Design and how they relate to each other.
![[image from here](http://uniknow.github.io/AgileDev/site/0.1.8-SNAPSHOT/parent/ddd/core/building_blocks_ddd.html)](imgs/domain_model_relation.png)


## Operation Flow

```

       presentation       │          Application                      domain                        Infrastructure
                                                      │                                   │
                             7                                 Data Model
               ┌──────────────────────────────┐       │       ┌────────────────────────┐  │     ┌─────────────────────┐
               │          │                   │               │ ┌─────────────┐        │        │   Network Service   │
               ▼             Bloc             │       │       │ │  Aggregate  │        │  │     └─────────────────────┘
        ┌─────────────┐   │ ┌─────────────────┴─────┐         │ └─────────────┘        │                   ▲
────────▶   Widget    │     │ ┌────────┐ ┌────────┐ │    2    │ ┌────────┐             │  │                │ 6
        └─────────────┘   │ │ │ Event  │ │ State  │ │────┬───▶│ │ Entity │             │        ┌─────────────────────┐
User           │            │ └────────┘ └────────┘ │ │  │    │ └────────┘             │  │     │ Persistence Service │
interaction    │          │ └──────▲────────────────┘    │    │ ┌─────────────────┐    │        └─────────────────────┘
               │                   │                  │  │    │ │  Value Object   │    │  │                ▲
               └──────────┼────────┘                     │    │ └─────────────────┘    │                   │ 5
                     1                                │  │    └────────────◈───────────┘  │     ┌─────────────────────┐
                          │                              │                 │contain             │    Unit of Work     │
                                                      │  │      ┌────────────────────┐    │     └─────────────────────┘
                          │                              │      │      Service       │                     ▲
                                                      │  │      └────────────────────┘    │                │
                          │                              │                                                 │ 4
                                                      │  │     Repository                 │                │
                          │                              │   ┌─────────────────────────────────────────────┴───────────────┐
                                                      │  │   │ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐  3  ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐ │
                          │                              └───┤         Interface        ────▶       Implementation         │
                                                      │      │ └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘     └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ │
                          │                                  └─────────────────────────────────────────────────────────────┘
                                                      │
```


1.  Widget accepts user interaction and translates the interactions into specific Events. The events will be sent to the Application layer, handled by the specific `bloc`. The `bloc` sends the states changed by the events back to the widget, and finally the `Widget` updates the UI according to the state. The pattern is depicted in this diagram. (More about the flutter [bloc](https://bloclibrary.dev/#/coreconcepts?id=bloc))
    ```
                       ┌────────────     State     ────────────┐
                       │                                       │
                       ▼                          Bloc         │
                ┌─────────────┐                  ┌─────────────┼─────────┐
       ────────▶│   Widget    │                  │ ┌────────┐ ┌┴───────┐ │
                └─────────────┘                  │ │ Event  │ │ State  │ │
    User interaction   │                         │ └────────┘ └────────┘ │
                       │                         └───────────────────────┘
                       │                                     ▲
                       │                                     │
                       └──────────     Event     ────────────┘

    ```

2.  The `bloc` processes the events using the services provided by the `Domain` layer.
    1. Convert DTO (Data Transfer Object) to domain model and Domain Model to DTO.
    2. Domain model is the place where all your business logic, business validation and business behaviors will be implemented.
       The Aggregate Roots, Entities and Value Objects will help to achieve the business logic.


3. Calling repositories to perform additional operations. The repositories interfaces are declared in the `Domain` layer and are implemented in the `Infrastructure` layer.
   You can reimplement the repository interface with different languages, such as `Rust`, `C++` or `Dart`. etc.
   ```
              Domain                       Infrastructure

     Repository A                │
    ┌────────────────────────────┼────────────────────────────────┐
    │ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐  │  ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐ │
    │         Interface        ──┼─▶       Implementation         │
    │ └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘  │  └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ │
    └────────────────────────────┼────────────────────────────────┘
     Repository B                │
    ┌────────────────────────────┼────────────────────────────────┐
    │ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐  │  ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐ │
    │         Interface        ──┼─▶       Implementation         │
    │ └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘  │  └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ │
    └────────────────────────────┴────────────────────────────────┘
   ```
4. The responsibility of [Unit of Work](https://martinfowler.com/eaaCatalog/unitOfWork.html) is to maintain a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems((No intermediate state)).
   If any one persistence service fails, the whole transaction will be failed so, roll back operation will be called to put the object
   back in initial state.

5. Handling operations (INSERT, UPDATE and DELETE) with SQLite to persis the data.
6. Saving or querying the data in the cloud to finish the operation.
